ASTM F1711-96

Standard Practice for Measuring Sheet Resistance of Thin Film Conductors for Flat Panel Display Manufacturing Using a Four-Point Probe

SCOPE
1.1 This practice describes methods for measuring the sheet electrical resistance of sputtered thin conductive films deposited on large insulating substrates, used in making flat panel information displays. It is assumed that the thickness of the conductive thin film is much thinner than the spacing of the contact probes used to measure the sheet resistance.  
1.2 This standard is intended to be used with Test Method F 390.  
1.3 Sheet resistively in the range of 0.5 to 5000 ohms per square may be measured by this practice. The sheet resistance is assumed uniform in the area being probed.  
1.4 This practice is applicable to flat surfaces only.  
1.5 Probe pin spacings of 1.5 mm to 5.0 mm, inclusive (0.059 to 0.197in. inclusive) are covered by this practice.  
1.6 The method in this practice is potentially destructive to the thin film in the immediate area in which the measurement is made. Areas tested should thus be characteristic of the functional part of the substrate, but should be remote from critical active regions. The method is suitable for characterizing dummy test substrates processed at the same time as substrates of interest.  
1.7 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.  
1.8 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.

General Information

• Status: Historical
• Publication Date: 31-Dec-1995
• Technical Committee: F01 - Electronics
• Drafting Committee: F01.17 - Sputter Metallization

Relations

• Replaced By: ASTM F1711-96(2002) - Standard Practice for Measuring Sheet Resistance of Thin Film Conductors for Flat Panel Display Manufacturing Using a Four-Point Probe
• Effective Date: 10-Dec-2002

Standards Content (Sample)

NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
Designation: F 1711 – 96
Standard Practice for
Measuring Sheet Resistance of Thin Film Conductors for
Flat Panel Display Manufacturing Using a Four-Point Probe
Method
This standard is issued under the fixed designation F 1711; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope 3.1.1 For definitions of terms used in this practice see Test
Method F 390.
1.1 This practice describes methods for measuring the sheet
electrical resistance of sputtered thin conductive films depos-
4. Summary of Practice
ited on large insulating substrates, used in making flat panel
4.1 This practice describes the preferred means of applying
information displays. It is assumed that the thickness of the
Test Method F 390 to measure the electrical sheet resistance of
conductive thin film is much thinner than the spacing of the
thin films on very large flat substrates. An array of four pointed
contact probes used to measure the sheet resistance.
probes is placed in contact with the film of interest. A measured
1.2 This standard is intended to be used with Test Method
electrical current is passed between two of the probes, and the
F 390.
electrical potential difference between the remaining two
1.3 Sheet resistivity in the range 0.5 to 5000 ohms per
probes is determined. The sheet resistance is calculated from
square may be measured by this practice. The sheet resistance
the measured current and potential values using correction
is assumed uniform in the area being probed.
factors associated with the probe geometry and the probe’s
1.4 This practice is applicable to flat surfaces only.
distance from the test specimen’s boundaries.
1.5 Probe pin spacings of 1.5 mm to 5.0 mm, inclusive
4.2 The method of F390 is extended to cover staggered
(0.059 to 0.197 in. inclusive) are covered by this practice.
in-line and square probe arrays. In all the designs, however, the
1.6 The method in this practice is potentially destructive to
probe spacings are nominally equal.
the thin film in the immediate area in which the measurement
4.3 This practice includes a special electrical test for veri-
is made. Areas tested should thus be characteristic of the
fying the proper functioning of the potential measuring instru-
functional part of the substrate, but should be remote from
ment (voltmeter), directions for making and using sheet resis-
critical active regions. The method is suitable for characteriz-
tance reference films, an estimation of measurement error
ing dummy test substrates processed at the same time as
caused by probe wobble in the probe supporting fixture, and a
substrates of interest.
protocol for reporting film uniformity.
1.7 The values stated in SI units are to be regarded as the
4.4 Two appendices indicate the computation methods em-
standard. The values given in parentheses are for information
ployed in deriving numerical relationships and correction
only.
factors employed in this practice, and in Test Method F 390.
1.8 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the
5. Significance and Use
responsibility of the user of this standard to establish appro-
5.1 Applying Test Method F 390 to large flat panel sub-
priate safety and health practices and determine the applica-
strates presents a number of serious difficulties not anticipated
bility of regulatory limitations prior to use.
in the development of that standard. The following problems
2. Referenced Documents are encountered.
5.1.1 The four-point probe method may be destructive to the
2.1 ASTM Standards:
thin film being measured. Sampling should therefore be taken
F 390 Test Method for Sheet Resistance of Thin Metallic
2 close to an edge or corner of the plate, where the film is
Films With a Collinear Four-Probe Array
expendable. Special geometrical correction factors are then
3. Terminology required to derive the true sheet resistance.
5.1.2 Test Method F 390 is limited to a conventional col-
3.1 Definitions:
linear probe arrangement, but a staggered collinear and square
arrays are useful in particular circumstances. Correction factors
This practice is under the jurisdiction of ASTM Committee F-1 on Electronics
are needed to account for nonconventional probe arrange-
and is the direct responsibility of Subcommittee F01.17 on Sputtered Thin Films.
ments.
Current edition approved June 10, 1996. Published August 1996.
5.1.3 Test Method F 390 anticipates a precision testing
Annual Book of ASTM Standards, Vol 10.04.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
F 1711
arrangement in which the probe mount and sample are rigidly
positioned. There is no corresponding apparatus available for
testing large glass or plastic substrates. Indeed, it is common in
flat panel display making that the probe is hand held by the
operator.
5.1.4 It is difficult, given the conditions cited in 5.1.3, to
ensure that uniform probe spacing is not degraded by rough
handling of the equipment. The phased square array, described,
averages out probe placement errors.
5.1.5 This practice is estimated to be precise to the follow-
ing levels. Otherwise acceptable precision may be degraded by
probe wobble, however (see 8.6.4).
5.1.5.1 As a referee method, in which the probe and
measuring apparatus are checked and qualified before use by
the procedures of Test Method F 390 paragraph 7 and this
practice, paragraph 8:
standard deviation, s, from measured sheet resistance, R ,is #
S
0.01 R
S
5.1.5.2 As a routine method, with periodic qualifications of
probe and measuring apparatus by the procedures of Test
Method F 390 paragraph 7 and this practice, paragraph 8:
standard deviation, s, from measured sheet resistance, R ,is #
S
0.02 R
S
6. Apparatus
6.1 Probe Assembly:
6.1.1 The probe assembly must meet the apparatus require-
ments of F 390, 5.1.1-5.1.3.
6.1.2 Four arrangements of probe tips are covered in this
practice:
6.1.2.1 In-Line, Collinear, Probe Tips, with current flowing
between the outer two probes (see Fig. 1A). This is the
conventional arrangement specified in Test Method F 390.
6.1.2.2 Staggered Collinear Probe Tips, with current flow-
FIG. 1 Four-Point Probe Configurations
ing between one outer and one interior probe (see Fig. 1B).
This arrangement is sometimes used as a check to verify the
results of a conventional collinear measurement (see 6.1.2.1). highly conductive boundary within6 0.25 mm (60.010 in.) is
6.1.2.3 Square Array, with current conducted between two
required. In most instances a vernier depth gage is suitable.
adjacent probe tips (see Fig. 1C). 6.4.2 Toolmaker’s Microscope, capable or measuring incre-
6.1.2.4 Phased Square Array, with current applied alter- ments of 2.5 μm.
nately between opposite pairs of tips (see Fig. 1D). This
7. Test Specimen
arrangement has the advantage of averaging out errors caused
by unequal probe spacing. 7.1 The test article shall be either a display substrate that has
6.1.3 Probe Support— The probe support shall be designed been sputter coated with the thin film of interest, or, alterna-
in such a manner that the operator can accurately lower the tively, a dummy plate coated in the same operation as the
probes perpendicularly onto the surface and provide a repro- substrate of interest.
7.2 The conductive film must be thick enough that it is
ducible probe force for each measurement. Spring loading or
gravity probe pin loading are acceptable. continuous. Generally this requires that the film be at least 15
nm (150Å ) thick.
6.2 Electrical Measuring Apparatus— The electrical appa-
ratus must meet the apparatus requirements of Test Method 7.3 The area to be tested shall be free of contamination and
mechanical damage, but shall not be cleaned or otherwise
F 390, 5.2.1 through 5.2.4.
6.3 Specimen Support— The substrate to be tested must be prepared.
supported firmly. 7.4 Note that a sputtered film may also coat the edge of the
6.4 Additional Apparatus: glass and can coat the back side of the substrate (“over spray”).
6.4.1 If measurements will be made within a distance of 20 Thus the edge of the glass cannot be automatically assumed to
times the probe spacing from an insulating or highly conduc- be insulating. If sheet resistance determinations will be made
tive edge or corner (20 3 S , where i 5 1, 2, 3, or 4, with within a distance of 20 times the probe spacing to an edge of
i
reference to Fig. 1), an instrument capable of measuring the the substrate it is necessary to ensure that the film terminates at
distance from the probe array position to the insulating or the edge.
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
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F 1711
7.4.1 To eliminate over spray error in compensating for
edge effects at an insulating boundary (see 10.2.2), either make
a fresh cut of the substrate, grind the edge to remove any
residual film, or etch the film from the edge.
7.4.2 Scribing the substrate near the edge using a glass
scribe is not a reliable remedy.
7.4.3 Use a simple 2-point probe ohmeter to verify that the
substrate edge is insulating.
7.5 Soda Lime Glass Substrates—Special precautions may
be required in measuring the sheet resistance of sputtered thin
films on soda lime glass substrates. The surface of this glass
can be somewhat electrically conductive (on the order of
1 3 10 V ) when the ambient relative humidity is about 90 %
or higher.
7.5.1 The glass conductivity degradation may interfere with
the sheet resistance measurement when specimen sheet resis-
tivity is 1000 V/square or higher.
NOTE 1—Set R 5 approximately the resistance measured on the speci-
7.5.2 Ensure that films >1000 V/square sheet resistance v
men film of interest as follows:
deposited on soda lime glass are conditioned at less than 50 %
R 5 R 5 R
a b v
humidity for at least 48 h prior to measurement, and that the
R 5 100 3 R .
d v
measurement is performed at an ambient relative humidity less
NOTE 2—Set I approximately the same as used for measurement of the
than 50 %.
specimen film of interest, typically 0.05 to 0.50 mA, so that V is
7.5.3 Note that at relative humidity less than 50 % the
comparable to that obtained in performing the sheet resistance determi-
surface resistance of soda lime glass in on the order of 1 3 10 nation.
NOTE 3—If R is set equal to a multiple of In2/2p for the in line probe
12V/ square. v
of Fig. 1A, or In2/2p for a square array, then the magnitude of V is the
sheet resistance value for an equivalent film measurement.
8. Suitability of Test Equipment
FIG. 2 Voltmeter Test Circuit
8.1 Equipment Qualification—The probe assembly and the
electrical equipment must be qualified for use as specified in
without changing I, make a second reading, V , with R shorted
a a
Test Method F 390, paragraphs 7.1 through 7.2.3.3 on suitabil-
(close switch CMR ), and finally complete a third reading, V ,
a b
ity.
with R shorted (open CMR , close CMR ). The common mode
b a b
8.2 Voltmeter Malfunctions—Modern solid state voltmeters
error is approximately as follows:
using field effect transistors in the signal input circuitry are
2 2 1/2
E 5 $1/2@~V 2 V! 1 ~V 2 V! # %/V 3 100 (2)
electrically fragile; failure of a field effect transistor degrades
cm a b
the input impedance. This failure mode is a particular hazard if
where:
input protection is not provided and if films with static charges
E 5 the percentage voltage error contributed by com-
cm
are probed. It is recommended that the error from the voltmeter
mon mode voltages. The voltmeter must be re-
input impedance be checked periodically using the test circuit
paired or replaced if E exceeds 0.5 %.
cm
illustrated in Fig. 2.
8.3 Voltage Limited Constant Current Supply—In cases of
8.2.1 Input Impedance Error—To measure the input imped-
high sheet resistance or high contact resistance, the voltage at
ance error, set the constant current, I, and take the voltage
the constant current source may not be high enough to drive the
reading, V. Then, without changing I, make a second reading,
set current. This condition causes very large errors in computed
V , with R shorted (close switch IMP, Fig. 2). The impedance
d d
sheet resistance.
error for R >> R is approximately as follows:
imp v
8.3.1 Ensure that the measuring circuit contains a direct
E 5 @~V 2 V!/V # 3 100 (1)
imp d d reading ammeter (see Test Method F 390, 5.2.4), permitting the
operator to verify the true current flow.
where:
8.3.2 Alternatively, provide electronic means to divide the
E 5 the percentage voltage error contributed by the
imp
measured voltage by the measured current. This ratio may be
finite voltmeter input impedance.
provided digitally or by a dual-slope integrating voltmeter with
8.2.2 Common Mode Rejection Error—State of the art
reference voltage inputs.
voltmeters typically have high common mode rejection (on the
8.4 Avoid Arcing On the Film—As the probes are making or
order of 90 dB), but this may be degraded by the failure of a
breaking contact with the film, the voltage driving the constant
field effect transistor in the input circuit (8.2). Reduction of
current source can cause arcing damage to the film and the
common mode rejection will cause errors in measuring sheet
probes. To avoid arcing, keep the constant current supply
resistance if unequal probe contact resistances contribute high
voltage low or provide switching preventing application of
common mode voltages. Common mode rejection error may be
current supply voltage until after contact is made with the film
measured using the test circuit shown in Fig. 2.
under test.
8.2.2.1 To measure the common mode rejection error, set
the constant current, I, and take the voltage reading, V. Then, NOTE 1—Ten-volt potential typically does not cause visible arcing
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
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F 1711
damage, but 100 volt potential often does.
8.5.4.2 The sheet resistance of the reference film may be
calibrated using a 2-point or 4-point method, using the bus bars
8.5 Fabrication and Use of Sheet-Resistance Reference
as contact lines. The measured V/I ratio
...